Sample electrification measurement method and charged particle beam apparatus

ABSTRACT

The present invention has the object of providing a charged particle beam irradiation method ideal for reducing the focus offset, magnification fluctuation and measurement length error in charged particle beam devices. To achieve these objects, a method is disclosed in the invention for measuring the electrical potential distribution on the sample with a static electrometer while loaded by a loader mechanism. Another method is disclosed for measuring the local electrical charge at specified points on the sample, and isolating and measuring the wide area electrostatic charge quantity from those local electrostatic charges. Yet another method is disclosed for correcting the measurement length value or magnification based on fluctuations found by measuring the amount of electrostatic charge at the specified points under at least two charged particle optical conditions, and then using a charged particle beam to measure fluctuations in measurement dimensions occurring due to fluctuations in the electrostatic charge at the specified locations.

This application is a continuation of application Ser. No. 11/429,237,filed on May 8, 2006, now U.S. Pat. No. 7,372,028 which is acontinuation of application Ser. No. 11/077,130, filed Mar. 11, 2005,now U.S. Pat. No. 7,087,899, which is a continuation of application Ser.No. 10/483,596, filed on Feb. 10, 2004, now U.S. Pat. No. 6,946,656,which are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

The present invention relates to a charged particle beam device andrelates in particular to a measurement method and device thereof forinspecting or measuring the dimensions and shape of a pattern formed ona sample piece.

BACKGROUND ART

The greater scale of integration and miniaturization of semiconductordevices in recent years has resulted in many diverse patterns beingformed on the wafer and makes it ever more important to evaluate andmeasure the dimensions and shapes of these patterns. How fast thesemeasurement points can be detected is critical for quickly andautomatically testing the numerous measurement points. During fastdetection of measurement points, it is necessary to focus on the patternafter shifting to the measurement point and to also set the desiredmagnification (scale) for observing that point.

In charged particle optical systems, the conditions for focusing on thewafer are determined by the acceleration voltage of the charged particlesupply, the voltage applied to the wafer, and the height of the wafer.

In the method disclosed for example in JP-A No. 126573/1999, a laserbeam is irradiated onto the wafer, the height of the wafer is detectedby using that reflected light, and the height information obtained inthis way is fed back to an objective lens control system serving as onecontrol device for the charged particle optical system, and thenecessary excitation voltage is applied to the objective lens at thesame time that movement to the observation point ends.

DISCLOSURE OF THE INVENTION

In recent years however, more and more wafers are being found to containa static electrical charge or electrostatic charge that still remainseven when the wafers are electrically grounded. The cause of this staticor electrostatic charge may for example be due to a fixed electricalpotential from splitting (split polarization) of polarized materialwithin the resist due to friction during applying of the resist coatingby a spin coater. Another possible cause is residual electrical chargesfrom etching that uses plasma.

These residual electrostatic charges remaining on the sample can causethe focus of the charged particle beam to deviate and are a cause ofmagnification fluctuations and measurement errors in the chargedparticle beam device. A method is disclosed for example in JP-A No.176285/1995 for resolving the focus deviation problem by storing a focusoffset value for each measurement point on a scanning electronmicroscope to prevent focus deviations from interfering with automaticmeasurement. Another method is disclosed in JP-A No. 52642/2001 forinstalling electrometers at multiple points in proximity to the sampleinside a vacuum and feeding a retarding voltage back as a value based onthose measurement results.

However, the technology disclosed in JP-A No. 176285/1995 has thefollowing problems. The electrostatic voltage on the wafer is determinedby the temperature and humidity, state of the resist and plasmaintensity in that manufacturing process, so the electrostatic voltage isnot a fixed value even on wafers undergoing the same manufacturingprocess. So even if the focus deviation is stored in a file for makingautomatic measurements, the focus deviation has to be updated(rewritten) for each wafer. A long time is therefore needed to measure awafer and this delay caused productivity to drop. The electrostaticelectrical potential also still remained unchanged on the wafer so thatthe actual accelerating voltage is different from the acceleratingvoltage actually needed. This differential causes differences incontrast and tiny structures to appear in secondary charged particlephenomenon that are formed. Other problems also still unresolvedincluded errors in controlling the magnification, etc.

In the method disclosed in JP-A No. 52642/2001, using electrometersinstalled within a vacuum, the electrostatic electrical potential cannotbe measured without moving to the measurement point so a long time wasrequired to make a measurement at one point. Another problem is thatwhen a breakdown occurred, the charged particle and stage in the vacuumunit has to be exposed to the outside air so that maintenance of theequipment is difficult. Yet another problem is that the multipleelectrometers have to be adjusted to constantly provide the same output.

A first object of the present invention is to provide a device andmethod for detecting the characteristic electrostatic charge state ofthe sample without having to also measure the electrostatic charge ateach measurement point.

A second object of the present invention is to provide a method idealfor reducing or eliminating measurement errors or fluctuations inmagnification due to electrostatic charges, a magnification adjustmentmethod, and a device to implement these methods.

To achieve the first object, a technique is proposed in the presentinvention for measuring the electrical potential distribution on thesample by utilizing a static electrometer to measure the voltage on thesample during movement of the sample being loaded by the loader of thecharged particle beam device.

To achieve the second object, a technique is proposed in the presentinvention for measuring electrostatic charges at specified points on thesample, and from that electrostatic charge quantity then isolating andmeasuring the wide area electrostatic charge. As another method toachieve the second object, the electrostatic charge quantity atspecified locations is irradiated under at least two charged particleirradiation conditions, and a fitting coefficient is formed thatexpresses changes in the electrostatic charge voltage from changes inthe irradiation conditions, and the pattern dimensions are thencorrected based on the feedback coefficient thus formed.

The best modes for carrying out the invention are described next indetail using the specific embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the overall structure of the scanningelectron microscope;

FIG. 2 is a drawing showing the relative positions of the wafer,conveyor arm, and static electrometer on the conveyor path;

FIG. 3 is a diagram showing the flow of the method for determining theelectrostatic charge distribution coefficient on the wafer surface;

FIG. 4 is a drawing showing the method for measuring the electrostaticvoltage on the wafer with multiple probes;

FIG. 5 is a drawing showing the entire structure of the SEM containing astatic electrometer and energy filter;

FIG. 6 is a drawing showing the entire structure of the SEM containingan energy filter and sample height measurement means;

FIG. 7 is a drawing showing the detailed structure of the energy filter;

FIG. 8 is a drawing showing the detailed structure of the electrostaticcorrection controller;

FIG. 9 is a drawing for describing the optical magnification by theobjective lens;

FIGS. 10A, 10B, and 10C are drawings for describing the mechanismcausing electrostatic charges on the sample;

FIG. 11 is a drawing describing the mechanism by which the electrostaticvoltage makes the magnification fluctuate;

FIG. 12 is a graph showing the relation of the wide area electrostaticvoltage and the magnification fluctuation sensitivity coefficient;

FIG. 13 is a graph showing the relation of the localized electrostaticvoltage and the magnification fluctuation sensitivity coefficient;

FIG. 14 is a graph showing the relation of the localized electrostaticvoltage and the irradiation area of the irradiation electron beam;

FIG. 15 is a graph showing the relation of the magnification fluctuationsensitivity coefficient and the magnification of the irradiationelectron beam;

FIG. 16 is a graph showing the relation of the predose magnification,actual measurement length value and estimated measurement length value;

FIG. 17 is a block diagram of the localized electrostatic chargecorrection section;

FIG. 18 is a flow chart showing the procedure for correcting thelocalized electrostatic voltage based on measuring the electrostaticcharge at two or more points;

FIGS. 19A and 19B are drawings showing the electrostatic charge stateduring predose magnification;

FIG. 20 is a block diagram of another localized electrostatic chargecorrection section;

FIG. 21 is a flow chart showing the procedure for correcting thelocalized electrostatic charge when correction data is available in thememory section;

FIG. 22A and FIG. 22B are drawings showing the error check screen forestimated measurement length from localized electrostatic correction;

FIG. 23 is a flow chart showing the procedure for a composite ofmultiple embodiments; and

FIG. 24 is an overall concept drawing of an SEM containing anultraviolet beam device.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

The embodiments of the present invention are described next whilereferring to the drawings. The example in the embodiment was describedas using a scanning electron microscope (SEM). However, the presentinvention is not limited to this and other charged particle beam devicessuch as ion beam irradiation devices can be used. The example in thepresent embodiment also describes detecting secondary electrons and/orreflected electrons which are one type of charged particle. However, thepresent invention is not limited to this and may detect other chargedparticles such as secondary ions, etc.

FIG. 1 shows the overall structure of the present invention. Anintegrated controller 42 controls the overall device via the chargedparticle optical system controller 41, stage controller 40, and waferconveyor 28, based on the observation position information, waferinformation and acceleration voltage of the charged particle entered bythe operator from the user interface 43.

The wafer conveyor 28 extracts the wafer from the wafer cassette 29using the conveyor arm 30 after receiving an instruction from theintegrated controller 42. The wafer conveyor 28 opens the gate valve 26b separating the sample exchange chamber 25 maintained in a vacuum froman external section connecting to the outer atmosphere. The waferconveyor 28 loads the wafer into the sample exchange chamber. The waferinserted in the sample exchange chamber is conveyed to a sample chamber24 via the gate valve 26 a and is clamped onto the sample stage 21.

The charged particle optical system controller 41 controls a highvoltage controller 34, a retarding controller 33, a condenser lenscontroller 35, an amplifier 36, an alignment controller 37, a deflectionsignal controller 44, and an objective lens controller 39 according toinstructions received from the integrated controller 42. A primarycharged particle beam 13 pulled from the charged particle supply 11 bythe pull-up electrode 12 is irradiated onto the wafer 19 after beingfocused by the condenser lens 14 and objective lens 18. During the aboveprocess, the path of the charged particle beam is aligned by thealignment coil 16. The upper part of the wafer is also scannedtwo-dimensionally by a signal received by the deflecting coil 17 from adeflecting signal controller via a deflecting signal amplifier 38. Inthe following description, a signal for changing the optical conditionsof the charged particle beam is sent to each optical element andcalculated in a section called a controller, control device or controlprocessor, etc.

A retarding voltage (negative voltage when using an electron microscope)is applied to the wafer from the retarding controller 33 to deceleratethe charged particle beam. The irradiating of the primary chargedparticle beam 13 onto the wafer 19 causes secondary charged electrons 20to be emitted from the wafer. These secondary electrons 20 are thentrapped by the secondary charged electron detector 15 and are used viaan amplifier as luminance signals for the secondary charged electrondisplay device 46. The secondary charged electron display devicedeflection signal is synchronized with the deflection signal from thedeflection coil so the pattern shape of the wafer is faithfullyreproduced on the secondary charged electron display device.

In order to test and observe the pattern on the wafer at high speed, asample stage detects the wafer height when the wafer has moved to thedesired observation point. The focus of the objective lens must then bealigned according to that height. A function is therefore installed inorder to detect that wafer height by using light. The sample stageposition detector 32 detects the position of the sample stage. At thepoint where the sample stage is close to the desired position, a heightdetection laser emitter 22 irradiates light towards the wafer. Thisreflected light is received by the position sensor 23 and the waferheight detected from that received light position. The amount of focusdetermined according to this detected height is then fed back to theobjective lens. The focus is therefore already set when the sample stagearrived at the specified position and the pattern can be automaticallydetected without the intervention of the operator.

If there is no electrostatic charge on the wafer, the excitation currentrequired for focusing the objective lens is generally expressed by thefollowing function (1).I _(obj) =F(V _(o) ,V _(r) ,Z)  (1)Here, I_(obj) is the excitation current for the objective lens whenthere is no electrostatic charge on the wafer, F is the function forcalculating the excitation current of the objective lens, V_(o) is thevoltage of the charged particle supply, V_(r) is the wafer electricalpotential, (retarding voltage applied to the wafer), Z is the height ofthe wafer. The function F can be derived by electron optical simulationor by actual measurement. A fixed focus control can be used to establisha relation shown in formula (1) for applying a retarding voltage with aelectrical potential equivalent to a wafer usually having noelectrostatic charge. However, when the wafer itself contains anelectrostatic charge then the excitation current value required by theobjective lens is as shown in formula (2). The focus current will differdepending on whether the wafer holds or does not hold an electrostaticcharge.I _(obj) ′=F(V _(o) ,V _(g) ′,Z)  (2)

Therefore, due to this difference the focus cannot be aligned no matterhow accurately the height is detected, so the secondary charged particleimage will appear blurred, detection at the observation point will failand automatic measurement will be impossible. Here I_(obj)′ is theexcitation current of the objective lens when the wafer holds anelectrostatic charge, V_(g)′ is the total voltage of the retardingvoltage V_(r) and the wafer electrostatic voltage ΔV_(g) or in otherwords, V_(g)=V_(r)+ΔV_(g).

The electrostatic charge on the wafers differs according to factors suchas the resist and the material in the underlayer but in most cases is ina concentric circular shape. The present invention measures the amountof electrostatic charge in this concentric circular shape on the waferand then uses this electrical potential as feedback. The wafer storedinside the wafer cassette is extracted by the conveyor arm 30 (conveyormechanism) and is measured by the probe 31 while being conveyed in thesample exchange chamber. The measured value is reported to the chargedparticle optical system device via the static electrometer 45.

In the example described in the present embodiment, the probe formeasuring the electrical potential on the sample is above the movementpath of the sample being conveyed by the conveyor mechanism andinstalled at a position separated from the material. However the presentinvention is not limited to this example. The probe for example may beinstalled on the movement path of the device for delivering andaccepting the sample in the preheat chamber from the sample chamber, orthe device for conveying the sample into the preheat chamber from theoutside.

In the above example, the wafers tended to have an electrostatic chargein a concentric circular shape. So the overall electrical potentialacross the entire sample can be found by measuring the electricalpotential distribution in a linear shape including the center positionon the wafer surface. The following description shows an exampleparticularly effective for measuring this kind of electrical potentialdistribution with a scanning electron microscope on a sample such as asemiconductor wafer.

FIG. 2 is a drawing showing the relative positions of the wafercassette, and the conveyor arm as an essential element of the sampleloader device, and the wafer, and static electrometer and sampleexchange chamber. The wafer is extracted by the conveyor arm 30 from thewafer cassette 29 and conveyed into the sample exchange chamber 25. Theprobe 31 of the static electrometer is clamped onto the clamp bed 53above the conveyor path of the wafer and further so that the center line52 aligns with the wafer center line 51 above the wafer. The staticelectrometer probe measures the voltage of both the wafer and thegrounded conveyor arm so that a more accurate value can be obtained bycalibrating the wafer electrical potential based on the ground potentialof the conveyor arm. The position the wafer will pass is a permanentlyfixed position, and since the probe is also clamped to the clamp bed,the relation of these two positions will not change so stablemeasurements can always be made. The probe is outside of the vacuum soeven if the probe becomes defective, it can easily be repaired orreplaced.

In the present embodiment, the probe was installed outside the vacuum tomake handling easier. However, the invention is not limited to this andthe probe may be installed anywhere along the path of the wafer. Also inthis embodiment, the wafer is moved so that the center of the probe isaligned with the center line of the wafer. However the present inventionis not limited to this example. As described above, the electrostaticcharge on the wafer is a concentric circular shape in most cases. Whenthe distribution of this electrostatic charge takes the form of aso-called peak, where the wafer center is the highest point and theelectrostatic charge becomes lower towards the edge of the wafer, evenif the probe center is somewhat offset from the centerline of the wafer,the overall electrical potential distribution can be determined. Theoverall electrical potential distribution can therefore also bedetermined from a linear shaped electrical potential distribution thatis offset from the wafer center.

FIG. 3 is a chart expressing the electrostatic charge voltage measuredfrom the surface electrical potential as a distribution coefficientabove the wafer surface. FIG. 3 also shows the retarding feedbackprocedure. The conveyor arm for the wafer does not usually operate at aconstant speed so even if the measurement time is a fixed periodtime-wise, the coordinates on the wafer will not be at fixed intervalsfrom each other. However an electrical potential corresponding toaccurate coordinates can be obtained if the coordinates on the wafer arecalculated from the speed pattern of the conveyor arm during theelectrical potential measurement. A distribution function for theelectrical potential can be made based on this acquired data. Anapproximate expression is first created as an even function (quarticfunction in FIG. 3) based on all of this acquired data.

Next, the differential at each measurement point versus this approximateexpression is calculated. The electrical potential measurement valuecontains an error. When this differential (value) is larger than anestablished threshold, it is excluded since the error in the measurementis large. An approximate expression is once again formed without theexcluded data. This process is repeated several times and ends when thedifferential for all values is smaller than the threshold. The functionmade in this way is a function expressing the distance from the centerof the wafer as the electrical potential.

The electrical potential for making the correction is calculated fromthis function, and from the stage coordinates acquired from the stagecontroller device. This correction voltage is supplied to the wafer viathe retarding controller shown in FIG. 1. Data is acquired each time thewafer under observation is conveyed to the sample exchange chamber. Thisdata is valid until wafer observation ends and an instruction to returnthe wafer to the original wafer cassette is issued.

The embodiment of the present invention was described above. In theembodiment of the invention, a method was described for feeding back themeasured electrostatic charge of the wafer unchanged, as retardingvoltage. However, the electrostatic charge voltage made be converted toan excitation current for the objective lens and fed back. In that casehowever, the retarding voltage and the wafer electrostatic chargevoltage added together should not exceed the voltage of the chargedparticle power supply. If the voltage of the charged particle powersupply for example is −2000 volts, then when the charged particlevoltage needed for beaming onto the sample is −300 volts, the retardingvoltage applied to the wafer must be −1700 volts.

Under these conditions, consider the case when observing a wafer havinga maximum electrostatic charge of −290 volts. Here, the primary chargedparticle beam can still reach the sample even if a voltage is applied asa retarding voltage to correct the −290 volt static charge, or even ifthat voltage is converted to an excitation current and applied to theobjective lens. However, on a wafer with a maximum electrostatic chargeof −310 volts, the combined retarding voltage and electrostatic voltagewill total −2010 volts thus exceeding the charged particle power supplyvoltage.

In that case, the primary charged particle beam will not be able toreach the sample and is reflected away. A voltage of 310 volts must beapplied as a retarding voltage to compensate for the −310 volts. Themeasured voltage may also be fed back to the charged particle powersupply instead of applying it as a retarding voltage. Also in theembodiment of the present invention, instead of using a magnetic fieldlens whose high inductance makes high speed control difficult as thefeedback destination for the retarding voltage, an electrostatic lensmay be installed as the objective lens, or an electrostatic lensseparately installed along with a magnetic field lens. A focuscorrection value based on the electrostatic charge voltage can then befed back to these static lenses.

Among other methods for aligning the focus, when the SEM employs theso-called boosting method wherein a positive voltage is applied totubular electrodes inside the objective lens, the focus can be alignedby adjusting this positive voltage. Most other technology for aligningthe focus of the electron beam may also be utilized.

In the present invention, one static electrometer probe is installed toalign with the center of the wafer; however, multiple probes may also beinstalled. FIG. 4 is a drawing of a structure for measuring the entirewafer surface with multiple probes arrayed along the wafer conveyancepath. Here, multiple probes 31 are arrayed in a matrix on the clamp bed.In this case, the wafer 19 is temporarily stopped at a specifiedposition along the conveyance path and the electrostatic charge measuredat the respective points. This method has the advantages that there isno need to worry about the relation between speed or coordinates sincethe conveyor arm has stopped. Another advantage is that a distributioncoefficient can be obtained even when the electrostatic charge does nothave a symmetrical distribution. Also, the measuring points have alreadybeen established so that during fully automatic inspection of thesemiconductor pattern width or fault inspection with the scanningelectron microscope, those measurement points or the electrostaticcharge near those points can be selectively tested and feedback thenapplied.

The present embodiment need not only use just feedback based on thequantity of electrostatic voltage, but may also combine it with otherinformation to find a feedback value for the retarding voltage. Further,when a problem has occurred in the static electrometer due to any numberof causes, and feedback is applied to the retarding voltage, converselythe focus value itself might then deviate. In such cases another meansmay be installed to evaluate the focus. When a problem then appears inthe focus evaluation value, then a means may also be installed toperform fault diagnosis of the static electrometer, stop the focusfeedback process based on the electrostatic charge measurement, and warnthe operator of the abnormality.

As explained above, the present invention is capable of correcting theelectrostatic charge even on wafers where focus offsets have occurreddue to electrostatic charges and the success (pass) rate for patterndetection during automatic measurement has dropped. The presentinvention is also capable of automatically measuring wafers in the sameway as wafers with no electrostatic charge. The invention further hasthe merit that the electrostatic charge voltage can be measured on eachwafer so that measurement files are not needed and also that the filedoes not have to be revised according to whether or not there is anelectrostatic charge or the size of that charge.

Second Embodiment

In view of the problems in making accurate tests and measurements inparticular when different electrostatic charge phenomenon occur in thesample (semiconductor wafer, etc.), the embodiment described nextrelates to a device and method allowing highly precise testing andmeasurement even when different electrostatic charge phenomenon.

In a charged particle beam device, output information from a secondarycharged particle detector is synchronized with the scanning by thecharged particle beam and reproduced on an image display device asdescribed above. The ratio of distance A between two points on thescanned image on the CRT (or display device) versus the distance abetween two points on the sample, is the observation magnificationM_(SEM).M _(SEM) =A/a

The distance a between two points on the sample is usually in inverseproportion to the observation magnification M_(SEM) since the screen onthe display device is a fixed size. By therefore measuring the distanceA between the two points on the scanned image on the display, anddividing A by the observation magnification M_(SEM), we can derive theline dimension as a=A/M_(SEM).

Along with the advances in miniaturization in the semiconductor industryin recent years, the SEM is being used in place of the opticalmicroscope in semiconductor fabrication processes or in testing afterthe fabrication process (for example, electrical operation tests ordimension measurements using the electron beam). In the sample (wafer)used by the semiconductor industry as the insulation, fluctuations inthe insulation are occurring over time due to irradiation by the primaryelectron beam and causing deterioration in the scanned image.

A typical technology to resolve this problem was disclosed in JP-A No.151927/1993 constituting a predose method wherein the SEM emitted(irradiated) a primary electron beam at a magnification different fromthe magnification during observation, and a static charge wasprogressively generated on the surface of the sample. A retarding methodand a boosting method were next developed as disclosed in JP-A No.171791/1997. In these methods, the retarding voltage applied to thesample was adjusted, and by observation with a primary electron beamhaving a low acceleration voltage below one kilovolt, a positive staticcharge was formed on the insulation. These methods generated a stablesurface static charge for recreating the image and further attained ahigh resolution of approximately 3 nanometers.

Following this, a method was developed utilizing a SEM as in JP-A No.200579/2000 wherein instead of a primary electron beam during the usualobservation, an energy electron beam was first irradiated (onto thesample) to progressively generate a surface electrostatic charge. Thesemethods allowed easily generating a stable, high surface electrostaticvoltage and permitted observations of electrical potential contrastbased on the difference in electrostatic charge voltage and the filmremaining on the bottom of contact holes with a high aspect ratio.

However, when observing under the condition of this surfaceelectrostatic charge voltage, it was found that a fluctuation of someseveral percent occurred in the measurement dimension values as thesurface electrostatic charge was increased. Due to ever shrinking sizesin the semiconductor process, these fluctuations in measurementdimensions exceeded their allowable thresholds. The cause of the problemwas fluctuations in observation magnification M_(SEM) accompanying thesurface electrostatic charge.

FIG. 9 is a concept drawing showing an electronic optical systemcomposed of a scanning deflector, objective lens and sample. This figureshows the relation of the coil current I₇ of scanning deflector 107 tothe optical magnification M_(obj) of objective lens 106 and observationmagnification M_(SEM). The primary electron beam 101 emitted radiallyfrom one point on the crossover surface focused on one point on thewafer 108 surface. When the emission point of an imaginary primaryelectron is separated by an amount 1 from the center axis using thescanning deflector 107, it deviates by M_(obj) on the sample surface.When the conversion coefficient of the scanning deflector 107 and thecoil current are respectively set as K and I₇, the distance a betweentwo points on the sample can be calculated with the next formula.a=KM_(obj)I₇  (4)Also, when the conversion coefficient of the CRT (display) is L, thedistance A between two points on the scanning image on the CRT is shownin the next formula.A=LI₇  (5)Here, considering the case where the optical magnification has shiftedfrom M_(obj) to M_(obj)′, the electrical current for scanning thedistance between two points a on the sample changes from I₇ to I₇′, andthe distance between two points A on the scanning image of the CRTchanges to A′.a=KM_(obj)I₇′  (6)A′=LI₇′  (7)The observation magnification consequently changes from M_(SEM) toM_(SEM)′.M _(obj)′=(M _(obj) /M _(obj)′)M _(SEM)  (8)Using the following formula allows making correct dimension measurementseven if the observation magnification has shifted.a=A′/M _(SEM)′  (9)Being able to calculate the optical magnification M_(obj) and M_(obj)′with good accuracy regardless of whether there is an electrostaticcharge, allows measuring dimensions with high accuracy.

FIG. 10A through 10C are drawings showing the principle of a surfaceelectrostatic charge on the wafer. The retarding voltage V_(r) isapplied to the wafer substrate. FIG. 10A shows the case where the waferhas a characteristic electrostatic charge prior to observation by SEM,because of friction from the spin coater applying the resist coating, orfrom etching with plasma. The electrostatic voltage in FIG. 10A spansthe entire surface of the wafer and is therefore called the wide areaelectrostatic voltage ΔV_(g). The wide area electrostatic voltage in thevicinity of the observation point is V_(g)=V_(r)+ΔV_(g). The opticalmagnification M_(obj) at wide area electrostatic voltage V_(g) isexpressed by the following formula (1).M _(obj) =M(V _(o) ,V _(g) ,Z)  (10)

The function M can be found by electronic optical simulation or byactual measurement. The electrostatic voltage ΔV_(s) from the electronbeam irradiation on the other hand, is localized as shown in FIG. 10Band is called a localized electrostatic voltage. When both electrostaticcharges overlap, the localized voltage in FIG. 10C isV_(s)=V_(g)+ΔV_(s).

FIG. 11 is a drawing showing the mechanism by which the wide areaelectrostatic voltage V_(g) and localized electrostatic voltage ΔV_(s)make the optical magnification M_(obj) of the objective lens change. Thewide area electrostatic voltage V_(g) varies the electrical potentialwithin the objective lens 106 a so that an electrostatic lens is formedon the sample and the focus deviates. When this focus is aligned, amarked change occurs in the excitation current I₆. This I₆ changes andalso the energy beamed onto the sample fluctuates so that energyconcentrates as in track 1 _(a), and the optical magnification M_(obj)fluctuates. Conversely however, V_(g) can be estimated from the amountof fluctuation in I₆.

The electrostatic voltage ΔV_(s) from the electron beam irradiation islocalized so there is almost no effect on the excitation current I₆.Regardless of this, the localized electrostatic voltage ΔV_(s) forms aminute static lens 108 b so that the primary electrons 101 areconcentrated along the track as in 101 b, and makes the opticalmagnification M_(obj) fluctuate greatly. The above description thereforeconfirms that the wide area electrostatic charge exerts a large effecton the focus and the localized electrostatic charge exerts a largeeffect on the magnification.

As shown above, the two electrostatic phenomenon have completelydifferent characteristics. The extent of the effect exerted on the focusand magnification by each electrostatic phenomenon is different so thathigh accuracy correction cannot be achieved even if correcting each ofthem separately is attempted.

To solve this problem, the wide area electrostatic voltage ΔV_(g) andthe localized electrostatic voltage ΔV_(s), can be isolated andmeasured, or a means to estimate them can be installed and a means tocalculate the correct optical magnification M_(obj) can then be achievedbased on this data.

Correcting the deflection intensity of the scanning deflector based onthe amount of magnification correction allows accurately displaying atwo-dimensional scanning image at the specified observationmagnification. Simplifying the magnification correction of themeasurement length value itself will prove effective in measurement ofdimensions in the semiconductor process.

The effect of the present invention is shown by referring to FIG. 12 andFIG. 13.

FIG. 12 shows the magnification fluctuation sensitivity coefficientT_(g) when the wide area electrostatic voltage V_(g) has fluctuatedwithin a range from −0.6 kV to −1.5 kV versus a retarding voltageV_(r)=−1.2 kV. The magnification fluctuation quantityΔM_(g)=(M_(obj)′−M_(obj)) can be calculated from T_(g) and ΔV_(g) by thefollowing formula.ΔM _(g) /M _(obj) =T _(g) *ΔV _(g)  (11)Here, the T_(g) fluctuated due to the wide area electrostatic voltageV_(g) and observation conditions prior to the electrostatic charge.Therefore each of these observation conditions found by calculation orexperiment per the graph of FIG. 8 must be stored. Also, instead ofusing the formula (11), the magnification M_(obj) or the M_(obj)′ may befound directly from the wide area electrostatic voltage V_(g).

On the other hand, FIG. 13 shows the magnification fluctuationsensitivity coefficient T_(s) when the beam irradiation area hasfluctuated at a retarding voltage V_(r)=−1.2 kV. The magnificationfluctuation amount ΔM_(s)=(M_(obj)′−M_(obj)) can be calculated fromT_(s) and ΔV_(s) by the next formula.ΔM _(s) /M _(obj) =T _(s) *ΔV _(s) /V _(acc)  (12)Here, T_(s) is the fluctuation due to the beam irradiation area size andobservation conditions prior to the electrostatic charge. The formula(12) shows a good proportional relationship with the magnificationcorrection ΔM_(s) and localized electrostatic voltage ΔV_(s). The T_(s)can be grouped into four sections according to the beam irradiation area(in other words, the beam magnification). A magnification lower than 50times is regarded as a wide area electrostatic charge. The section from50 times to 500 times is a transition region from a wide area staticcharge to a localized static charge. The section from 500 times to 5,000times is regarded as largely a fixed value. A high magnification from5,000 times shows a trend for T_(s) to gradually diminish. Therefore,one side of the irradiation area may preferably be from 10 μm to 300 μm,so as to contain a section where the magnification fluctuationsensitivity coefficient T_(s) includes a section with a largely fixedvalue from 500 to 5,000 times. This kind of section allows maintainingthe estimated accuracy of the correction value and reduces the number ofdata that must be stored in advance.

FIG. 5 shows a first working example of the SEM of the presentembodiment. The primary electron beam 101 from the cathode (negativeelectrode) 104 is focused by a condenser lens 105, and two-dimensionalscanning of the wafer 108 further performed by the scanning deflector107. The primary electron beam 101 applies a negative retarding voltageto the wafer 108 via the sample stage 109 so that the beam isdecelerated in the decelerating magnetic field between the objectivelens 106 and the wafer 108, and the beam on the wafer 108 is narrowedeven further by the lens action of objective lens 106.

Secondary electrons 102 are emitted when the primary electron beam 101irradiates onto the wafer 108. The magnetic field created between theobjective lens 106 and the wafer 108 functions as an acceleratingmagnetic field on the secondary electrons 102 that were generated topull these secondary electrons 102 into electron beam passage holes ofobjective lens 106 and these secondary electrons 102 then rise whilesubject to the lens effect rendered by the magnetic field of objectivelens 106. These rising secondary electrons 102 strike the conversionelectrode 110 with high energy, to newly generate secondary electrons103. These secondary electrons 103 are pulled towards the scintillator111 that was applied with a positive voltage of approximately 10 kV.Light is emitted when the secondary electrons 103 strike thescintillator 111. Though not shown in the drawing, this light issupplied to a photoelectron multiplier tube via a light guide, convertedinto electrical signals, and after being amplified, the output is usedfor brightness modulation of the CRT.

The explanation of FIG. 5, described the control processor as beingintegrated with the scanning electron microscope, or a subsection of themicroscope. Needless, to say, the invention is not limited to thisexample, and a separately installed control processor as described nextmay be utilized instead of integrated with a scanning electronmicroscope. In that case, a notification medium for conveying thedetection signal detected by the secondary electron detector to thecontrol processor, and conveying the signal from the control processorto the deflector or lens of the scanning electron microscope isrequired. An input/output terminal is also needed for input or output ofthe signal conveyed by that notification medium. Further, a controlprocessor to install a program for implementing the following describedprocessing in a storage medium, and comprising a means for supplying thenecessary signals to a scanning electron microscope having an imagememory, and also executing that program may be used.

The device of the present embodiment contained a static electrometer asdescribed for example in the first embodiment as a measurement means(voltage differential measurement device) for measuring the wide areaelectrostatic voltage ΔV_(g). The wide area electrostatic voltage on thewafer has a concentric circular shape so that the electrical potentialdistribution of the entire sample can be known by measuring theelectrical potential distribution in a linear shape including the centerposition on the wafer. Therefore the method as described for the firstembodiment wherein a static electrometer probe 114 is clamped along theconveyance path of the wafer 108, and the movement of the conveyor arm181 to measure along a linear shape is applicable. The wide areaelectrostatic voltage ΔV_(g) is expressed as a function of the distancer from the wafer center by utilizing the measurement data, and each themeasurement point is moved, a voltage V_(r) is fed back for theretarding voltage. Also, the voltage that the primary electron beam 101beams onto the wafer 108 is generally made a fixed voltage valueV_(acc)V₀+V_(g). Here, V₀ is equivalent to the voltage of the cathode104.

This embodiment also contains a secondary electron energy filter as ameasurement means (voltage differential measurement device) for thelocalized electrostatic voltage ΔV_(s). A mesh electrode 112 forexample, is installed below the conversion electrode 110. The voltageapplied by this mesh electrode 112 is swept using the wide areaelectrostatic voltage V_(g) as a reference point, and the signalconversion quantity of the secondary electrons (so-called S curve)measured.

The S curve at the observation point of the actual sample and the Scurve measured on a conductive sample surface are compared, and theshift voltage set as the localized electrostatic voltage ΔV_(s).

The electrostatic correction controller 120 measures the wide areaelectrostatic voltage V_(g), and executes an S curve measurementsequence up to acquiring of a localized electrostatic voltage ΔV_(s) Theamount of magnification compensation is then calculated based on theexcitation current for the objective lens 106 and the V_(g) and ΔV_(s)that were found, and the deflection intensity of the scanning deflector107 then corrected.

In view of the fact that a localized static charge exerts a large effecton magnification compared to the wide area electrostatic charge, thepresent embodiment corrects the magnification by subtracting a valueequivalent to the wide area electrostatic charge, from an electrostaticcharge (localized electrostatic charge) at a specified location. Inmeasuring electrostatic charges merely by using an energy filter, thelocalized and wide area electrostatic charges (at least an area largerthan the scanning area, for example an area larger than an observationarea with a magnification of 50 times) are detected in a compoundedstate. So the present embodiment, by subtracting the electrostaticcharge at the electron beam scanning locations measured by staticelectrometer 114, from the electrostatic charge measured by the energyfilter, the localized electrostatic change can be measured based on theactual electron beam without depending on the wide area charge.

This embodiment also allows adjusting the deflection range of thescanning deflector based on the magnification fluctuation quantityΔM_(s) acquired from the above described calculation method. Thisembodiment also allows correcting the measured length (or endmeasurement) value.

When adjusting the deflecting range of the scanning deflector and thatscanning deflector is the electromagnetic type, the electrical currentrequired for correcting the magnification fluctuation quantity ΔM_(s),can be added to or subtracted from the original deflection current tomake the adjustment. An accurate measurement length value can also becalculated by multiplying or dividing the magnification fluctuationratio by the measurement length acquired by a measurement length methodused in scanning electron microscopes of the known art and using theresult for feedback to the measurement length value. In the presentembodiment, the wide area electrostatic charge and localizedelectrostatic charge were measured while isolated from each other,however methods for adjusting the scanning deflector and methods forcorrecting the measurement length are not limited to this method.

FIG. 6 shows a second working example of the present embodiment. In thisexample, a means to measure the sample height has been added instead ofthe static electrometer of the previous working example. For example, alaser emission device 115 for detecting the sample height at the pointin time that the sample stage 109 has approached the specifiedmeasurement point, beams a laser light 116 towards the wafer 108. Aso-called Z sensor here is a position sensor 117 receives that reflectedlight and detects the wafer height from the position that the light wasreceived. The wide area electrostatic voltage V_(g) is determined fromthis data on the sample height and excitation current of the objectivelens when exactly focused so that if the relation of these threephysical quantities are calculated by test or by an electronic opticalsimulation, then the wide area electrostatic voltage V_(g) can beestimated without having to directly measure the voltage.

In this embodiment, the electrostatic correction controller 120 executesan S curve measurement sequence until the localized electrostaticvoltage ΔV_(s) is obtained and sample height measurement with the Zsensor are obtained for estimating the wide area electrostatic voltageV_(g). Further, the magnification correction quantity is calculatedbased on the excitation current for the objective lens and by the V_(g),ΔV_(s) found the same way as in the previous working example, and thedeflection intensity of the scanning deflector 107 or the acquiredlength value is corrected.

A different working example of the embodiment is described next. Thisexample is an SEM comprising the static electrometer and the sampleheight measurement means of the two previous working examples. Sincethis working example contains these two means, the wide areaelectrostatic voltage V_(g) and localized electrostatic voltage ΔV_(s)can be measured with even high accuracy and greater stability.

In other words, if the first approximation value V_(g(1)) found from themeasurement data of static electrometer probe 114 or by the appropriateexpression, and the objective lens excitation current for exact focusestimated and combined with the sample height data from the Z axissensor, then the exact focusing task (so-called auto-focus) can becompleted in a short time. An accurate wide area electrostatic voltageV_(g) can be calculated from the differential between the excitationcurrent of the autofocus that was found and the excitation current ofthe objective lens calculated from V_(g(1)). If the V_(g) is correct,then the ΔV_(s)=ΔV_(s)−V_(g) which is the differential versus thelocalized surface voltage V_(S) can be accurately calculated, and themagnification correction will have greater accuracy.

FIG. 7 is a more detailed view for describing the energy filter for theabove embodiment. A mesh electrode 112 is installed enclosed from aboveand below by the grounded mesh electrode 113 and the secondary electronconversion electrode 110 above it. The mesh electrode 112 voltage isswept using the wide area electrostatic voltage V_(g) or the firstapproximation value V_(g(1)) as reset values. The S curve (secondaryelectron distribution when the voltage applied to the energy filter ischanged) is then measured. The grounded mesh electrode 113 prevents themagnetic field of the mesh electrode 112 from unwanted expansion towardsthe conversion electrode 110, etc. A fixed quantity of secondaryelectrons 102 strikes the lower mesh electrode 113 without requiring thevoltage of the mesh electrode 112, and create a fixed quantity of newsecondary electrons 130. These secondary electrons 130 are attractedtowards the scintillator 131 to which a positive voltage ofapproximately 10 kilovolts has been applied. The S curve can be measuredwith high accuracy by standardizing the current I₁₁ from thescintillator 111 with the current I₃₁ from scintillator 131. Images canbe displayed on the CRT the same as the case with the scintillator 111.

FIG. 8 is a drawing for describing in more detail the electrostaticcorrection controller 120 for the above three working examples. Thiselectrostatic correction controller 120 is composed of a staticelectrometer data table 201, an autofocus controller 202, a wide areastatic electrometer processor 203, an energy filter voltage controller204 for automatically measuring the S curve, a localized electrostaticvoltage processor 205, and a magnification correction processor 206.

First of all, data on the voltage V₁₄ for coordinates of the samplemeasured by the static electrometer or the fitting coefficient arestored in the static electrometer data table 201. The corresponding widearea electrostatic voltage ΔV_(g) is measured each time the observationpoint is moved, and the retarding voltage V₉ (=V_(r)) to the samplestage 109 is adjusted so as to satisfy the desired acceleration voltageV_(acc)=V₀+ΔV_(g)+V_(r). The autofocus controller 202 calculates theexcitation current I₆₍₁₎ for the acceleration voltage V_(acc) set withthe sample stage height data Z₁ from the Z sensor, and by sweeping thevicinity of this electrical current, search for the excitation currentI₆ for an exact focus. Next, when there is a differential between I₆₍₁₎and I₆, the wide area static electrometer processor 203 decides that anerror has occurred in V_(acc), and corrects the ΔV_(g), to find anaccurate wide area electrostatic voltage V_(g).

The energy filter voltage controller 204 on the other hand, measures theS curve in a non-charged state, and stores it in the localizedelectrostatic voltage processor 205. In the S curve measurementsequence, the applied voltage V₁₂ of mesh electrode 112 is swept usingthe wide area electrostatic voltage V_(g) or its estimated valueV_(g(1)), as a reference just as described above, and changes in theelectrical current I₁₁ of the secondary electrons are measured. Theelectrical current I₃₁ from the scintillator 31 can also be standardizedhere. The data to be stored may be data that was already processed suchas the S curve itself, or filter voltages in excess of a threshold,filter voltages with a maximum S curve slope. The S curve variessomewhat depending on the sample material so data may also be recordedfor each sample so that calculation accuracy can be enhanced from thenonwards. The localized electrostatic voltage processor 205 selects the Scurve to be used as the reference, and calculates the localizedelectrostatic voltage ΔV_(s) from the amount of voltage shift. Finally,the magnification correction processor 206 uses the respective formulas(1) and (2) from the wide area electrostatic voltage V_(g) and localizedelectrostatic voltage ΔV_(s) to calculate the magnification correctionamounts ΔM_(g) and ΔM_(s). By then correcting the electrical current I₇of the scanning deflector with the inverse of the total magnificationM+ΔM_(g)+ΔM_(s), an image can always be observed at the desiredmagnification regardless of the electrostatic voltage.

An effective method for boosting the processing speed when automaticallyprocessing large numbers of wafers on a semiconductor production line,is to reduce the number of S curve measurements by the energy filter.With an identical circuit pattern, and identical material, the localizedelectrostatic voltage ΔV_(s) will be the same (for each wafer) so aΔV_(s) that was already measured can be utilized. In some cases, one Scurve measurement for each wafer will also suffice. When a new S curveis measured, it is automatically added to the database of localizedelectrostatic voltage processor 205.

In the present embodiment, the fluctuation in the magnification rate canbe calculated with high accuracy for dimension measurement and imageobservation of the insulation material of the sample. Also, fluctuationsin the measurement length value can be corrected by setting a fixeddesired magnification rate or magnification change. Dimensions can inthis way be controlled with high accuracy in the currentlyultra-miniaturized semiconductor fabrication process.

A supplementary result also obtained is that image quality is stabilizedsince the energy of the primary electron beam irradiation onto thesample can be controlled to a high degree of accuracy. Further, bymonitoring the localized electrostatic voltage ΔV_(s), the destructionof the dielectric (insulation) by excessively large electrostaticcharges can be prevented, and an electrostatic voltage or index thereofcan be obtained for bottom surface observation via large aspect ratiocontact holes.

Third Embodiment

The localized electrostatic voltage ΔV_(s) varies the opticalmagnification M_(obj) of the objective lens as described using FIG. 11.The electrostatic voltage ΔV_(s) is localized due to electron beamirradiation so there is almost no effect on the excitation current I₆.Regardless of this, the localized electrostatic voltage ΔV_(s) forms aminute electrostatic lens 108 b. This lens causes the track 101 a of theprimary electron beam to be deflected by the global (wide area)electrostatic charge so as to concentrate onto the track 101 b and makethe optical magnification M_(obj) greatly fluctuate as described in theprevious embodiment.

Yet another method is described next for making accurate tests andmeasurements that are otherwise difficult due to different, overlappingelectrostatic phenomenon.

The present embodiment proposes a method for correcting themagnification fluctuation using the localized electrostatic voltageΔV_(s) and calculating the correct optical magnification M_(obj).

The magnification fluctuation brought about by the localizedelectrostatic charge is dependent on the localized electrostatic voltageΔV_(s) The localized electrostatic voltage ΔV_(s) is dependent on theelectron beam irradiation magnification (in the present embodiment, thisis hereafter called the predose magnification, mainly in order todescribe electron beam irradiation prior to using electron beam fortesting and measurement) M_(pre) and magnetic field near the samplesurface and the type of sample.

FIG. 14 shows the localized electrostatic voltage ΔV_(s), when thepredose magnification M_(pre) was varied at boosting voltages of 0.5 kVand 5 kV. The boosting referred to here is a method for installing acylindrical electrode to be applied with a positive voltage within theobjective lens so that the electron beam within the objective lens canat least reach a high acceleration to pass through the objective lensFIG. 14 shows the results when the surface electrical potential wasmeasured after varying the predose magnification while a voltage of 0.5kV was applied to the cylindrical electrode, and while 5 kV was applied.This boosting technology is disclosed in detail for example in JP-A No.171791/1997 (U.S. Pat. No. 5,872,358).

When the predose magnification and the sample surface electrical fieldare used as parameters for varying the localized electrostatic voltageΔV_(s), then the localized electrostatic voltage ΔV_(s) can becalculated in the following fitting function from the boosting voltageV_(b), retarding voltage V_(r), fitting coefficients A₁ and a₁, andpredose magnification M_(pre) parameters.ΔV _(s) =A ₁(V _(b) −V _(r))/M _(pre) +a ₁  (13)

Also, the magnification fluctuation quantity ΔM/M_(obj) can becalculated from ΔV_(s) using the magnification sensitivity coefficientT_(s).ΔM/M _(obj) =T _(s) *ΔV _(s)  (14)FIG. 15 shows the magnification fluctuation sensitivity coefficientT_(s), when the beam irradiation area (∝1/predosemagnification=1/M_(pre)) was varied at a retarding voltage of V_(r)=−1.2kV. T_(s) can be grouped into four sections according to the beamirradiation area. A section with a low magnification rate below 50 timesis regarded as a global electrostatic charge.

A section from 50 times up to 500 times is a transition region from theglobal electrostatic charge to a localized electrostatic charge. Asection from 500 times up to 5,000 times is regarded as largely fixed. Asection with a high magnification from 5,000 times upward has a tendencyfor the T_(s) to diminish. Therefore, if the magnification fluctuationsensitivity coefficient T_(s) of the irradiation area is set as anirradiation area (1 side is from 10 μm to 300 μm) equivalent to amagnification of 500 times to 5,000 times regarded as a fixed area, thenthe number of pre-stored data can be reduced while still maintaining theestimated correction value accuracy.

When the true value and actual measured value of the pattern dimensionsare respectively set as L, L_(ex), the magnification fluctuationquantity B=ΔM/M_(obj), can be calculated from the following formula.L/L _(ex)=1+B  (15)When estimating the true measured length using formula (13), formula(14), formula (15), the unknown coefficients are A₁ and a₁. Therefore,if the (V_(b)−V_(r)) proportional to the electrical field of the samplesurface or the predose magnification M_(pre), is changed and resultsfrom measuring two or more points are utilized, then the truemeasurement length L value can be estimated.

This method has the advantage that when observing an unknown insulationsample, the true measurement length can be estimated by changing thecharge location of the sample surface or the predose magnificationM_(pre), and measuring two or more different localized electrostaticvoltage ΔV_(s). Also, when using this method, instead of a fittingcoefficient having a predose magnification and surface charge locationas electrostatic variable parameters to vary the localized electrostaticvoltage ΔV_(s), as shown in formula (13); the same results can beobtained with another fitting coefficient having the energy of the inputbeam, irradiation time and electrons within the sample and the degree ofhole movement as the charge variable parameters.

By storing fitting coefficients a₁ and A₁ in the memory, true dimensionvalues can be estimated by using the measurement length value for onepredose magnification and surface electrical field. The fittingcoefficient a₁ used in formula (13) on the other hand, is not dominatedby the predose magnification and surface electrical field. Therefore, bysubstituting in the formula (13), formula (14) and formula (15) forirregularities in the a₁ utilized when correcting the measurement lengthof the same type of sample, the reliability of the adjusted parametersused to make the correction can be evaluated by means of the deviationin measurement length.

FIG. 16 is a graph showing the relation of the measurement length valuebefore correction to the measurement length value after correctionversus predose magnification. By storing the magnification fluctuationamount B for each predose magnification calculated from the truedimension values and measurement length before correction, the truedimension value can be estimated from the measurement length value ofone observation condition.

When performing the predose, a high contrast image can be obtained byutilizing the optimal acceleration voltage shown in JP-A No. 200579/2000and higher accuracy measurement results can be obtained.

A function for estimating the true dimension values (per the means ofthe first working example of the embodiment) by utilizing themeasurement length value of multiple points where the charge variableparameters for varying the localized electrostatic voltage were changed,is described next in an example using electrostatic correctioncontroller 120 of FIG. 5 and FIG. 6.

FIG. 17 is a block diagram of the electrostatic correction controller120. The electrostatic correction controller 120 is comprised largely ofa global electrostatic correction section 302 and localizedelectrostatic correction section 303. The localized electrostaticcorrection section 303 sets the measurement conditions (charge variableparameters, acceleration voltage, and primary electron beam irradiationtime during predose) via 313 a.

The measurement length measured per the conditions that were set isinput from an input device (not shown in drawing) via 313 b to thelocalized electrostatic correction section 303. A magnificationfluctuation amount B for correcting the localized electrostatic chargebased on the measurement conditions that were set and the measurementlength that was input, are input via 313 d to the electrostatic chargecorrector unifier section 304. Also, the magnification fluctuationamount calculated in the global electrostatic correction section 302 isalso input to the electrostatic charge corrector unifier section 304 via313 e.

The dimensions whose varied measurement length was corrected by theeffect of the global electrostatic charge and localized electrostaticcharge, was output from the magnification fluctuation amount derived inthe respective correction section of global electrostatic correctionsection 302 and localized electrostatic correction section 303 that wereinput from the electrostatic charge corrector unifier section 304.

FIG. 18 is a flow chart showing the process for correcting themeasurement length value. First of all, the charge variable parametersand measurement conditions are set in step s101. Next, in step s102, theelectron beam irradiates the sample to create an electrostatic chargeaccording to the conditions set in step s101. In step s103, themeasurement length value L_(ex) is acquired by measurement under thecharge variable parameters established in step s101 or step s109. Instep s104, a decision is made whether the measurement length L_(ex)acquired in step s103 has sufficient accuracy. When decided themeasurement length was not sufficiently accurate, the observationcondition settings of step s101 are corrected.

In step s106, a decision is made whether data has been collected enoughtimes for correction in step s107. If there is not enough data, thendifferent charge variable parameters are set in step s109 andmeasurement length again measured. In step s107, the measurement lengthvalue is corrected by using the measurement length value measured instep 105 and the charge variable parameters established in step s102 andstep s109. The measurement length value corrected in step s108 is outputto the monitor.

By using the localized electrostatic correction in the presentembodiment, the true dimension value can be estimated with high accuracyby making two or more measurements with different localizedelectrostatic voltages, even on samples of materials and shapes thathave had no preliminary measurement. Further, the measurement speed isimproved because no preliminary measurement with an energy filter isrequired for each magnification.

FIG. 19A to FIG. 19B show drawings of sample electrostatic charges whenthe predose magnification was changed and the sample given anelectrostatic charge. During length measurement at respectivemagnifications using two or more different predose magnifications, astable localized electrostatic charge can be quickly formed by using thefollowing procedure.

The sample 108 hold two types of electrostatic charges; a global (widearea) electrostatic charge V_(g) spanning the entire surface and alocalized electrostatic voltage ΔV_(s) created by the electronirradiation. In FIG. 19A, a residual electrostatic region 108 d can beformed when the predose magnification is raised during observation afterthat predose magnification was observed in a small state. The localizesstatic charge correction is badly effected unless sufficient time istaken for the charge on the residual electrostatic region tosufficiently weaken. However if the predose magnification is loweredafter observation of a large predose magnification as shown in FIG. 19B,then there is no residual electrostatic region, so measurement can startimmediately after the predose ends since no weakening time is required.Using the above procedure allows rapid observation with good accuracy inan electrostatic region.

The second working example of the embodiment of the present invention isdescribed next while referring to FIG. 20 and FIG. 21. In thisembodiment, the memory section 301 in the electrostatic correctioncontroller 120, contains a database of fitting constants for functionsexpressing the magnification fluctuation amount B or localizedelectrostatic voltage ΔV_(s). Measurement conditions (charge variableparameters, acceleration voltage, and primary electron beam time duringpredose) from localized electrostatic correction section 303 via 313 aare set here.

The measurement length value measured under the preset conditions, isinput via 313 b to the localized electrostatic correction section 303.The magnification fluctuation amount B or the charge variable parametersare input to the memory section 301 via 313 g. The ΔV_(s) matching thecharge variable parameters input in memory section 301 and fittingcoefficient linked to the variable change parameters or themagnification fluctuation amount B are input to the localizedelectrostatic correction section 303 via 313 h. After correction of themeasurement length value calculated using the data that was input, themeasurement length value is output via 313 d.

FIG. 21 is a flow chart showing the measurement procedures when storingthe correction data. The charge variable parameters and measurementconditions are set in step s201. Next, in step s202, the electron beamirradiates the sample to create an electrostatic charge according to theconditions set in step s201. In step s203, the measurement length valueL_(ex) is acquired by measurement under the charge variable parametersestablished in step s201 or step s209. In step s204, a decision is madewhether the measurement length L_(ex) acquired in step s203 hassufficient accuracy. When decided the measurement length was notsufficiently accurate, the observation condition settings of step s209are corrected.

In step s206, fitting coefficients for showing the magnificationfluctuation amount B or the V_(s), derived previously under the samecharge variable parameters as correction data are loaded from the memorysection 301. In step S207, a decision is made if correction is possibleor not from the measurement length value L_(ex) that was acquired andfrom the charge variable parameters established in step s201 or steps208. When decided that correction is impossible, the observationconditions are reset in step s209.

In step s208, the measurement length value L_(ex) measured in step s205is input to the localized electrostatic correction section 303. Themeasurement length value is at the same time obtained after correctionby the localized electrostatic correction section 303, using this data.

Performing localized electrostatic correction using this embodiment,allows shortening the time required for measurement length since thislocalized electrostatic correction can be performed from a measurementlength value measured under one charge variable parameter for a samplemeasured once and having at least the same pattern and same condition.If the optimal predose conditions such as shown in JP-A No. 200579/2000in step s201 are set, then stable measurements can be made with highaccuracy.

In the third working example of the present embodiment, in order toincrease the reliability of the corrected measurement length, the memorysection 301 contains a database holding fitting coefficients for fittingcoefficients for magnification fluctuation amount B or V_(s), andmeasurement conditions of the same type sample previously measured.

In this embodiment by utilizing a memory section 301 containing theabove described database, the measurement length value can bequantitatively evaluated by means of the differential in accuracy afterlocalized electrostatic correction. A threshold value is set from thedifferential of this measurement length value. If a measurement lengthexceeding this threshold setting is measured, then this measurement isjudged as abnormal and a decision is made whether the cause of theabnormal measurement is effects from impurities on the sample surface oran abnormal electrostatic charge, etc.

The procedure used in this embodiment is shown next. First of all, theprocedure for constructing the database is shown. The localizedelectrostatic voltage ΔV_(s) of the sample is changed, and adjustedcoefficients for localized electrostatic voltage at multiple points arederived from measurement length values measured between the same points.The fitting coefficients for the fitting coefficients of localizedelectrostatic voltage ΔV_(s), for the same type sample from betweendifferent two points are found in the same way.

Among the multiple fitting coefficients found by repeating this process,the irregularities of fitting coefficient a₁ not dependent on the chargevariable parameters are extracted. Irregularities of fitting coefficienta₁ and irregularities of the fitting coefficient a₁ corrected with thelength measurement differential are stored in the memory section 301.

Next, the procedure for deciding if there is an abnormal electrostaticcharge is shown by using the database that was formed. When themeasurement length value derived from the changing the charge variableparameter and measuring the measurement length (each time the sample isreplaced or a length measurement made) exceeds the threshold value foundfrom the differential with the stored length measurement value in thememory section 103, then a screen display as shown in FIG. 22A and FIG.22B appears and the user is notified of an abnormal electrostaticcharge.

When the fitting coefficient a₁ currently utilized in this correction iswithin the thresholds found from the irregularities of the previouslymeasured fitting coefficient a₁ stored in memory section 301, this showsthere is no abnormality and the localized electrostatic charge is judgedto be normal.

When the fitting coefficient a₁ currently utilized in this correctionexceeds the thresholds found from the irregularities of the previouslymeasured fitting coefficient a₁ stored in the memory section 301, thisshows that an abnormal electrostatic charge has occurred. Utilizing thisembodiment therefore allows knowing whether a localized electrostaticcharge is abnormal or not so that the length measurement can be foundwith a high degree of reliability.

The fourth working example of the present embodiment combines thefunctions of all the above embodiments. A flow chart of the process ofthe present embodiment is shown in FIG. 23. In step s1, a decision ismade whether or not there is correction data in the memory section 301for the current observation sample. When the correction data needed forthe current measurement length does not exist (no correction data), theprocess in the flow of loop 1 in step s100 shown in the first workingexample is performed to derive the post-correction measurement lengthvalue L.

In step s120 and step s160 the correction results are shown on a screen,and whether or not the currently used correction data will be used fromthe next time onwards is decided. If to be used from the next timeonwards, then the correction data is stored in the memory section 301 instep s170. In step s1 when there is correction data, the flow of loop 2starts and the processing shown in the second working example isperformed to derive a post-correction measurement length L. Thecorrection results displayed on the screen area shown in FIG. 22A andFIG. 22B.

When the evaluation shown in the third working example is made and anabnormal electrostatic charge is detected in step s210, a warning isdisplayed and loop 3 starts. By repeatedly performing the procedureshown in the first working example multiple times, a fitting coefficienta₁ is output under multiple conditions. In step s300, an fittingfunction is made using an average value for irregularities in themultiple fitting coefficients found in step s100.

The reliability of the currently formed fitting function is evaluatedfrom the differential in measurement lengths from irregularities inmultiple fitting coefficient a₁. If decided that the fitting function isnot reliable, then the process returns once again to step s100. Whendecided in step s300 that a fitting function was obtained that issufficiently reliable versus abnormal electrostatic charges, thecalculation results for the measurement length differential andpost-correction measurement length values using the fitting functioncurrently made in step s120 are displayed on the screen.

By performing localized electrostatic correction using the presentembodiment, during length measurement of the same patterns cut into thesame insulation piece sample, the measurement length process can beperformed at higher speeds and with more uniform accuracy.

The fifth working example of the present embodiment described hereutilizes a scanning electron microscope comprising an ultraviolet beamdevice 314 for minimizing effects on the previously measuredelectrostatic charge. FIG. 24 is a block diagram showing the scanningelectron microscope comprising an ultraviolet beam device 314. Thereference number 113 in the drawing denotes the input device forentering the measurement conditions. By irradiating the sample with anultraviolet beam from the ultraviolet beam device 314 for eachobservation, the electrostatic charge accumulated on the sample from theprevious measurement can be reset so that stable measurement ofdimensions can be performed.

The embodiments can therefore calculate the amount of fluctuation inobservation magnification with high accuracy for making dimensionmeasurements and image observation of the insulation sample. Thedimensions in the currently ultra-miniaturized semiconductor fabricationprocess can in this way be controlled in a short time with highaccuracy.

1. A method for measuring an electrical potential of a sample with ascanning electron microscope, comprising: preparing a functionindicating a relation between a value related to magnification of thescanning electron microscope and an electrical potential of a beamirradiation area by the scanning electron microscope; and calculatingthe electrical potential of the beam irradiation area based on theprepared function and the magnification being set.
 2. The method ofclaim 1, wherein the function is fitting function.
 3. The method ofclaim 1, wherein the function is prepared for each electrical fieldcondition of the sample.
 4. The method of claim 3, wherein theelectrical field condition is an electric potential difference betweenthe sample and an electrode arranged in an objective lens which focusesan electron beam.
 5. The method of claim 4, wherein the electrode issupplied a positive voltage.
 6. The method of claim 5, wherein theelectric potential difference is difference between the positive voltageand a negative voltage supplied to the sample.
 7. The method of claim 1,wherein the prepared function is represented as follows:ΔV _(s) =A ₁(V _(b) −V _(r))/M _(pre) +a ₁; and wherein ΔV_(s) is theelectrical potential of the beam irradiation area; A₁ is a fittingcoefficient; V_(b) is the positive voltage supplied to the electrodearranged in the objective lens; V_(r) is the negative voltage suppliedto the sample; M_(pre) is the magnification being set; and a₁ is afitting coefficient.
 8. A scanning electron microscope comprising: ascanning deflector for deflecting an electron beam; a processor forcontrolling the scanning detector by setting a magnification; whereinthe processor calculates an electrical potential of a beam irradiationarea based on a function indicating a relation between a value relatedto the magnification being set and an electrical potential of a beamirradiation area.
 9. The scanning electron microscope of claim 8,wherein the function is a fitting function.
 10. The scanning electronmicroscope of claim 8, wherein the function is prepared for eachelectrical field condition of a sample.
 11. The scanning electronmicroscope of claim 10, wherein the electrical field condition is anelectric potential difference between the sample and an electrodearranged in an objective lens which focuses an electron beam.
 12. Thescanning electron microscope of claim 11, wherein the electrode issupplied a positive voltage.
 13. The scanning electron microscope ofclaim 12, wherein the electric potential difference is differencebetween the positive voltage and a negative voltage supplied to thesample.
 14. The scanning electron microscope of claim 8, wherein thefunction is represented as follows:ΔV _(s) =A ₁(V _(b) −V _(r))/M _(pre) +a ₁; and wherein ΔV_(s) is theelectrical potential of the beam irradiation area; A₁ is a fittingcoefficient; V_(b) is the positive voltage supplied to the electrodearranged in the objective lens; V_(r) is the negative voltage suppliedto the sample; M_(pre) is the magnification being set; and a₁ is afitting coefficient.